The most customary way of obtaining such a frothy emulsion is to pour the desired amount of milk into a container, to immerse the steam outlet pipe in said container, shaking it up and down to get in the air necessary to form the froth. The quality of the froth obtained depends on the skill of the user who, when not a professional, may also be subject to splashing. For hygiene reasons, it will be appreciated that the pipe and the container containing the milk need to be cleaned after each use.
In order to try to obtain a more uniform quality of froth, various types of venturi-effect devices have been proposed in order to act somewhat as an interface between the steam outlet of an expresso coffee machine and a container containing milk.
The simplest type of venturi-effect device, described for example in patent U.S. Pat. No. 4,800,805, consists in an air carrying tube secured to the steam outlet pipe and having its opening positioned below said outlet, the entity having to be immersed in a container containing the desired amount of milk. The improvement described in patent U.S. Pat. No. 5,335,588 consists in securing the air line to a sleeve which can itself be fitted to the steam outlet pipe, the entity still having to be immersed in a container containing the milk.
Patent EP 0 243 326 describes a venturi-effect accessory that can be fitted to an expresso coffee machine, comprising a great many parts making it possible, in an aspiration chamber, to have an arrival of pressurized steam which, through a venturi effect, entrains air along a first duct and entrains along a second duct milk arriving from a reservoir incorporated into the machine or aspirated from a standard packaging by means of an immersed connecting dip tube. This mixture is then injected into a mixing chamber before leaving in the form of a froth.
An improvement proposed in patent U.S. Pat. No. 5,265,519 corresponds to an accessory of simpler design, with fewer parts to be assembled to form the venturi-effect nozzle, but still comprises an immersed connecting dip tube for carrying the milk. The device further comprises an anti-splash cap at the frothy preparation ejection orifice.
Patents EP 0 803 219 and EP 0 803 220 B1 also describe a device for preparing a frothy milk or cappuccino by pumping by means of two tubes immersed in a container of the cardboard carton type and connected to a venturi-effect device. Such a device is designed to accept a large-capacity container and therefore needs a refrigeration system in order to preserve the milk.
In all the cases, the immersed connecting dip tubes need to be cleaned after each “cappuccino”. It will also be seen that the accessory, which is not designed to be disposable, requires periodic upkeep if the production of cappuccinos are spaced-out, and that the amount of cappuccino produced from one occasion to the next is also dependent on the care taken by the operator.